In the manufacture of yarn, particularly yarn from synthetic fibers, there have been substantial developments in the art of false-twist and self-twist yarns because of various production advantages which can be realized using these techniques, and because such processes provide a shortened manufacturing route to a finished yarn product, and are therefore more economical as compared with conventional spinning and twisting processes.
As used herein, the term "false-twist" refers to a yarn in which a yarn strand is twisted at some intermediate point generating opposite twists on either side of the twist insertion device, with the point at which the device is located containing zero twist, which point will be referred to as a "node." The directions of twist are referred to as "S-twist" or "Z-twist," the appropriate letter being employed for twists in which the helices in twisted strands correspond with the middle portion of the appropriate letter.
The term "self-twist" is applied to yarns wherein two or more false-twisted strands are brought together and permitted to ply themselves. Approximately equal torsional forces of the same direction are stored in each of a pair or more of singles yarns which are later brought into contact. Torque is released, permitting the singles yarns to untwist, and in so doing, wrap around each other, forming a plied yarn.
Generally speaking, false-twisting and self-twisting and the yarns produced thereby have received considerable attention in recent years and reference is made to the following documents in which these yarns, the techniques for producing them, and specific apparatus related thereto are discussed:
"Self-Twist Yarn," D. E. Henshaw, Merrow Publishing Co., Ltd., Watford, Herts, England, 1971.
U.S. Pat. Nos.
Re. 27,717 -- Breen et al. PA0 3,225,533 -- Henshaw PA0 3,306,023 -- Henshaw et al. PA0 3,353,344 -- Clendening, Jr. PA0 3,434,275 -- Backer et al. PA0 3,507,108 -- Yoshimura et al. PA0 3,717,988 -- Walls PA0 3,775,955 -- Shah PA0 3,940,917 -- Strachan
While this is by no means an exhaustive listing of patents or literature references on this subject, the foregoing techniques describe part of the prior art.
As will be recognized from these and other references relating to this art, there are a number of problems inherent in producing yarn using self-twist techniques, these problems being related in part to the fact that the yarn tends to be relatively unstable due to the different twists in singles being able to cancel each other through the node area. In this regard, the above-cited U.S. Pat. No. 3,434,275, to Backer et al. suggests joining regions of twist reversal.
An improved apparatus for forming and self-twisting yarns and for joining self-twisted singles yarns at their nodes is disclosed in U.S. Pat. application Ser. No. 755,671, now U.S. Pat. No. 4,074,511, which application is assigned to the same assignee and is hereby incorporated by reference.